With a recent increasing social demand for environmental conservation, biodegradable polymers which are decomposed by microorganism have attracted attention. Specific examples of the biodegradable polymers include fusion-moldable polyesters including aliphatic polyesters such as polybutylene succinate, polycaprolactone and polylactic acid, and aliphatic/aromatic copolymer polyesters such as terephthalic acid/1,4-butanediol/adipic acid copolymers. Among these aliphatic polyesters, the polylactic acid, which is widely distributed in nature and is harmless to animals, plants and humans, is highly heat-resistant with a melting point of 140 to 175° C. The polylactic acid is promising as a less expensive thermoplastic biodegradable resin.
Where the polylactic acid is molded or formed into a sheet or a container, however, crystals of the polylactic acid are generally completely fused due to thermal history experienced during the molding or the forming, so that the resulting molded or formed article is poor in heat resistance.
Many attempts to impart the polylactic acid with heat resistance have been reported. For example, JP-A-8-193165 proposes a method for producing a molded article by injection-molding, blow-molding or compression-molding a polylactic acid polymer to which talc, silica or calcium lactate is added as a crystal nucleus agent. However, this method is problematic in that the crystallization of the polymer is insufficient without a heat treatment and the productivity is low with a lower crystallization speed of the polymer. Further, JP-A-4-220456 proposes a method in which polyglycolic acid and its derivative are added as a crystal nucleus agent to poly-L-lactide to increase the crystallization speed for reduction of an injection molding cycle time and to improve the mechanical characteristics of the resulting molded article.
However, JP-A-8-193165 states that an attempt was made to produce a molded article by the injection molding by the method stated in JP-A-4-220456 but failed at a mold temperature of not lower than Tg as disclosed in JP-A-4-220456.
JP-A-11-106628 discloses a method in which wax is employed as a crystal nucleus agent and a crystallization promoter, and a molded article is heat-treated at a crystallization temperature or kept in a mold set at the crystallization temperature for a predetermined period. However, the wax employed as the crystal nucleus agent is generally less compatible with the polylactic acid thereby to be bled out. Therefore, only a small amount of the wax is added, which is insufficient for formation of crystal nuclei.
JP-A-9-25345 discloses a method for imparting heat resistance and shock resistance without the use of a crystal nucleus agent, wherein an unstretched sheet is stretched 1.5 to 5 times for improvement of the crystal orientation and crystallinity of the sheet. However, the sheet produced by this method is a stretched sheet, which is further stretched when subjected to a forming process to provide a formed article. Unfortunately, the sheet once stretched is inferior in drawability and, therefore, is not suitable for deep drawing. The applications of the sheet are inevitably limited.